Duplexer and method of isolating an Rx-band and a Tx-band

ABSTRACT

A duplexer ( 1 ) with two band-pass filters ( 5, 6 ) comprising film bulk acoustic wave resonators (FBAR) ( 8, 9, 10, 11, 12, 13 ) has an extra antiresonant circuit in order to block the transmission signal. It has an extra resonant circuit in order to allow the desired receive signal to pass. The antiresonant circuit comprises the first FBAR ( 11 ) and a parallel inductor ( 14 ). The resonant circuit comprises the first FBAR ( 11 ) and a series inductor ( 7 ).

The invention relates to electronic devices enabling mobilecommunications or other wireless standards. In such systems bandseparating units are used to separate several bands. Examples thereofinclude duplexers, triplexers and multiplexers. The band separating unitmay be used to separate various transmit bands, or various receive bandsor several transmit and receive bands. For reasons of clarity thefollowing discussion will focus on the duplexer separating a receive anda transmit band. However, this is not meant to be limiting.

A duplexer provides the necessary isolation between the receive andtransmit bands. To avoid loading the Rx-filter on the Tx-filter in thisduplexer a 90° phase shifter is used to transform the low impedance ofthe Rx-filter in the Tx-band to high impedance. The Rx-filter thereforeacts as an open circuit in the Tx-band and does not load or influencethe behavior of the Tx-filter. The 90° phase shifter can be implementedas lumped inductors and capacitors or as a λ/4-transmission line.Well-known practical examples are the HPMD-7904 and HPMD-7905 duplexersin which the 90° phase shifter is implemented as a λ/4-transmissionline.

The invention also applies to front-end modules used for the receptionin multi-band ‘phones where the different bands are separated byfiltering. In these front-end modules the different Rx-filters can be aload to each other. Different topologies can be conceived to avoid thisloading. Essentially however, they can all be considered filternetworks.

EP-A2 0 962 999 relates to resonator structures of radio communicationapparatus. At least one resonator structure and one switch structure ofthis radio communication apparatus is manufactured on the same substrateduring the same process. When using bridge type bulk acoustic wave (BAW)resonators and micro-mechanical switches the same process steps as thoseused for creating the bridge structures can be used to create themicro-mechanical switch structure. The integration of switch structuresand resonators on the same substrate allows the manufacture of the verycompact filter and resonator structures needed for multi-system mobilecommunication means. According to EP-A2 0 962 999 BAW resonators can beintegrated on substrates, which are commonly used for active circuitry,such as silicon and gallium arsenide surfaces, wherein the switches arerealized by transistor structures using, for example, MESFETtransistors.

U.S. Pat. No. 6,262,637 B1 discloses an FBAR-based duplexer thatcomprises a first port, a second port, a third port, a first band-passfilter connected between the first port and the third port and a seriesarrangement connected between the second port and the third port. Thefirst band-pass filter includes a ladder circuit having shunt and serieselements. Each of the elements of the first ladder circuit comprises afilm bulk acoustic resonator (FBAR). The series arrangement includes a90° phase shifter in series with a second band-pass filter. The secondband-pass filter includes a second ladder circuit having shunt andseries elements. Each of the elements of the second ladder circuitcomprises a film bulk acoustic resonator. A band-pass filter comprisingshunt elements and series elements in which the series elements and theshunt elements are connected to form a ladder circuit, and each of theelements includes a film bulk acoustic resonator (FBAR). The 90° phaseshifter may comprise of lumped inductors and capacitors or aλ/4-transmission line. The use of integrated FBAR areas comprising anumber of interconnected FBARs fabricated on a common piezoelectriclayer as the transmit FBAR area and the receive FBAR area enables thefabrication of very small duplexer embodiments.

The main drawbacks of known devices is the size of the elements neededto tune the bulk-acoustic wave resonators in the filter. Generally,λ/4-transmission lines are used as 90° phase shifter. These striplineshave a very large size, which places severe constraints on the overallsize of the filter and duplexer in particular, and the electromagneticcoupling of the λ/4-transmission with the rest of the circuitry, whichcontributes significantly to the stop band behavior.

Thus, to fulfill the requirements of continuing miniaturization, oneobject of the invention is to provide an electronic device with a bandseparating unit that is small, can withstand high power levels and hassufficiently steep filter characteristics to create a narrow stop band.Another object of the invention is to provide a method of isolating anRx-band and a Tx-band.

The invention provides an electronic device comprising a band separatingunit with a first port, a second port and an antenna port. A firstband-pass filter connects herein the first port and the antenna port. Aseries arrangement connects the second port and the antenna port. Thisseries arrangement comprises a second band-pass filter that includes aladder circuit with shunt and series elements, of which series elementsat least a first element comprises a film bulk acoustic wave resonator(FBAR). The device of the invention is herein characterized in that thefirst element of the second band-pass filter has a parallel inductor,and in that the series arrangement comprises a frequency tuning elementbetween the antenna port and the second band-pass filter.

In the invention use is made of an element in the second band to blockthe first band. A frequency tuning element is provided to open thesecond band at the desired frequency spectrum. The first band is hereinunderstood to mean the connection between the antenna port and the firstport via the first band-pass filter. The second band is hereinunderstood to mean the series arrangement between the antenna port andthe second port. This element is the combined block of a bulk acousticwave filter and a parallel inductor. This will be further explained.

A normal bulk acoustic wave resonator has both a resonant and anantiresonant frequency. The resonator's resonance is its frequency ofminimum impedance and its antiresonance is its frequency of maximumimpedance. The parallel inductor of the inventive duplexer, togetherwith the static capacitance of its bulk acoustic resonator, forms aquasi extra antiresonant circuitry in the first band, e.g. that bandlocated between the first port and antenna. Thus it forms a shunt LCnetwork. This extra antiresonant circuit is preferably tuned tocorrespond to the center frequency of the first band and thus acts as anopen circuit in order to block that band.

As a result of the inventive arrangement of the invention, there is noneed to use phase shifters such as λ/4 transmission lines between thesecond band-pass filter and the antenna port. A frequency tuning elementis sufficient.

In a preferred embodiment, the frequency tuning element is a seriesinductor. The series inductor, together with the static capacitance ofthe first element of the second band-pass filter, forms a resonantcircuit in the second band. This resonant circuit is preferably tuned tocorrespond to the center frequency of the second band. The seriesinductor adds the resonant frequency of the second band to the bulkacoustic wave resonator in order to allow the desired second band topass. This resonant frequency is also included in the first band. Ingeneral, the series inductor is preferred. However, with higherfrequencies, a well-chosen interconnect may have the same function.Alternatively, particularly for a band-separating unit with more thantwo bands, other filters, such as high- or low-pass filters may be usedinstead of the series inductor.

In a further embodiment the band separating unit is a duplexer, whereinthe first port is a transmit port and the second port is a receive port.The resulting duplexer is small and does not have the disadvantage ofthe electromagnetic coupling of the λ/4 transmission line to the rest ofthe circuitry. Besides, bands that lie apart by a small guard band only,can be separated as well. An important example hereof is the USPCS CDMA1900 MHz system, with transmit bands from 1850–1910 MHz and receivebands from 1930–1990 MHz.

In another further embodiment, the band separating unit is a triplexerand the device is further provided with a third band-pass filter betweenthe antenna port (4) and a third port, which third band-pass filterincludes a ladder circuit with shunt and series elements, of whichseries elements at least a first element comprises a film bulk acousticwave resonator (FBAR) and is provided with a parallel inductor, andwherein a further frequency tuning element is present between theantenna port (4) and the third band-pass filter. Tests have shown thatthe inventive arrangement can be applied with success in such atriplexer as well. This triplexer is for instance used to separate threebands within the GSM standard system. Herein the same constraintsregarding size and functioning are present.

The first and second band pass filter preferably comprise ladder filterswith shunt and series elements. In order to be able to integrate theseinto one band separating unit, it is preferred that for all shunt andseries elements film bulk acoustic wave resonators are used. However,not necessarily, and discrete bulk acoustic wave resonators or discretesurface acoustic wave resonators can be applied alternatively. BAWs canbe used as modulators for both amplification and phase modulated systemsin smaller and increasingly complicated handheld units and demonstrate agood tolerance of high power levels. The desired support of severaldifferent standards and telecommunication systems requires several setsof filters and other radio frequency (RF) components. Surface acousticwave (SAW) resonators are very small in size but cannot withstand thesame high power levels as BAW. Besides, the frequency range whereinBAW-resonators can be used, extends higher frequencies (10–100 GHzinstead of 2–3 GHz) too much.

The invention is however applicable to lattice filters as well. In thiscase, the ladder circuit of the second band pass filter comprises thefirst element, its parallel inductor and one shunt element. The latticefilter is then provided in a series arrangement between this laddercircuit and the second port.

The bulk acoustic wave resonators are preferably made according tosilicon technology. These silicon dice (2 dice for a duplexer, 3 dicefor a multi-band phone) are mounted on a substrate carrier where theyare wire-bound or flip chipped. This is usually a cheap technology suchas FR4 or any other multi-layer laminate technology, such as LTCC (LowTemperature Coefficient Ceramics) for example. The inductors areintegrated on the substrate carrier.

It is also feasible for the parallel inductor to be integrated in thebulk acoustic wave resonator.

It is advantageous that the substrate carrier comprises furtherelectrical elements, and particularly that it constitutes a front-endmodule comprising at least one amplifier. The front-end module can besold as a complete building block to telephone manufacturers, but alsoto customers that do not have RF knowledge and nevertheless desire tointegrate RF functionality into an apparatus, such as a car. Generallysuch front-end modules comprise a power amplifier and/or a low-noiseamplifier and one or more impedance matching network. By furtherpreference more power amplifiers and low noise amplifiers are present,e.g. for each band an amplifier. Further on, an antenna and atransceiver IC as well as a voltage-controlled oscillator may be parthereof.

The inventive duplexer may for example be used in a USPCS CDMA 1900 MHzsystem, in a 2 GHz, 2.5 GHz or 3 GHz system or in a multi-band ‘phone,that includes, for instance a W-Lan or Bluetooth band.

The invention also provides a method. Devices of the personalcommunication system (PCS) that use Code Division Multiple Access (CDMA)operate in frequency bands of approximately 1,900 MHz. The guard bandbetween the portions of the spectrum assigned to the transmit signal andthe receive signal amounts to only approximately 1% of the carrierfrequency, i.e. 20 MHz. The bandwidth of the portions of the spectrumassigned to the transmit signal and the receive signal amount toapproximately 3% of the carrier frequency, i.e. 60 MHz. This means thatthe center frequencies of the Rx-band and Tx-bands are 40 MHz above andbelow the carrier frequency.

These and other aspects of the invention will become apparent from andwill be elucidated with reference to the embodiments describedhereinafter, where

FIG. 1 is a block diagram illustrating an inventive duplexer,

FIG. 2 is a graph illustrating the impedance of the series arrangementof the duplexer.

FIG. 1 is a block diagram illustrating an inventive duplexer 1comprising a transmit port 2, a receive port 3 and an antenna port 4, afirst band-pass filter 5 connecting the transmit port 2 and the antennaport 4, and a series arrangement connecting receive port 3 and antennaport 4, wherein the series arrangement comprises a second band-passfilter 6 and a series inductor 7. The transmit port 2 may be connectedto the output of a transmit apparatus (not shown), the receive port 3 tothe output of a receive apparatus (not shown) and the antenna port 4 toan antenna (not shown). The transmit/receive apparatus and the antennamay be part of a communications means such as a cellular phone or anyother means which uses a duplexer with two filters. The first band-passfilter 5 including a first ladder circuit has shunt 10 and serieselements 8, 9, the second band-pass filter 6 including a second laddercircuit also has shunt 13 and series 11, 12 elements, wherein theelements 8, 9, 10, 11, 12 and 13 of both the first and the secondband-pass filter 5, 6 comprise a film bulk acoustic wave resonator(FBAR). The second ladder circuit resonator 11 connected to the seriesinductor 7 has a parallel inductor 14. The antiresonant circuit isconstituted by the first FBAR 11 of the second band-pass filter 6 andforms an open circuit in the Rx-filter in the Tx-band. Thus, in effect,the open circuit causes the Tx-filter not to see the Rx-filter in theTx-band. The number of elements required in the branches isdesign-dependent. 3–5 branches are preferable.

FIG. 2 is a graph illustrating the impedance in % of the band-passfilter 6 of the duplexer series arrangement connecting receive port 3and antenna port 4. The course of the graph pertaining to the firstresonator 11 with inductor 14 is: capacitive impedance—high impedance(quasi-extra antiresonance in the Tx-band)—low impedance (seriesresonant frequency in the Rx-band)—high impedance (anti-resonantfrequency stop band Rx)—capacitive impedance. The graph shows threecharacteristic points:

-   1. a first peak A of approximately 100% at the center frequency    f_(Tx) of the transmission band, i.e. the stop band,-   2. a first minimum B of approximately 0% at the center frequency    F_(Rx) of the receive band, i.e. of the pass-band which is also the    resonator 11 frequency of series resonance,-   3. a second peak C of approximately 100% at the resonator 11    frequency of parallel resonance.

Maximum impedance A is achieved when the series arrangement of band-passfilter 6 is tuned to correspond to the center frequency f_(Tx) of thetransmission band, i.e. when the duplexer series arrangement acts as anopen circuit. The peak A mainly derives from the capacitance of theresonator 11 together with the parallel inductor 14. The value ofinductor 14 has to be carefully chosen in order to ensure that the opencircuit occurs at the center frequency f_(Tx) of the transmission band.

Minimum impedance B is achieved when the resonant circuit is tuned tocorrespond to the center frequency f_(Rx) of the receive band. Theminimum impedance B derives from the resonator 11 characteristic and isits frequency of series resonance. The second maximum impedance Cderives from the resonator 11 characteristic and is its frequency ofparallel resonance f_(p).

The series inductor 7 influences and improves the roll-off of thefilter.

The invention may be summarized as a band separating unit, preferably aduplexer 1, including two band-pass filters 5, 6 composed of film bulkacoustic wave resonators (FBAR) 8, 9, 10, 11, 12, 13 with an extraantiresonant circuit in order to block the transmission signal. It hasan extra resonant circuit in order to allow the desired receive signalto pass. The antiresonant circuit comprises the first FBAR 11 and aparallel inductor 14. The resonant circuit preferably comprises of thefirst FBAR 11 and a series inductor 7.

1. An electronic device, comprising a band separating unit forseparating a transmission band and a receive band comprising: a firstport, a second port and an antenna port; a first band-pass filterconnecting the first port and the antenna port; and a series arrangementconnecting the second port and the antenna port, the series arrangementcomprising: a second band-pass filter that includes a ladder circuitwith shunt and series elements, of which series elements at least afirst element comprises a film bulk acoustic wave resonator (FBAR), thefirst element of the second band-pass filter having a parallel inductor;and a frequency tuning element coupled between the antenna port and thesecond band-pass filter: wherein the first element and the parallelinductor together form an antiresonant circuit that blocks thetransmission band at the entrance of the second band-pass filter (6), asits antiresonant frequency corresponds to the center frequency f_(Tx) ofthe transmission band, and the first element together with the frequencytuning element form a resonant circuit that allows the receive band topass, as its resonant frequency corresponds to the center frequencyf_(Rx) of the receive band.
 2. An electronic device as claimed in claim1, characterized in that the frequency tuning element is a seriesinductor.
 3. An electronic device as claimed in claim 2, characterizedin that the series inductor, together with the static capacitance of thefirst element of the second band-pass filter, forms a resonant circuitin the receive band.
 4. An electronic device as claimed in claim 3,characterized in that this resonant circuit is tuned to correspond tothe center frequency of the receive band.
 5. Use of an electronic deviceas claimed in claim 2 in a USPCS CDMA 1900 MHz system, in a 2 GHz, 2:5GHz or 3 GHz system or a multi-band phone.
 6. An electronic device asclaimed in claim 1, characterized in that the band separating unit is aduplexer, wherein the first port is a transmit port and the second portis a receive port.
 7. An electronic device as claimed in claim 1,characterized in that the parallel inductor, together with the staticcapacitance of the first element of the second band-pass filter, formsan antiresonant circuit in the transmission band.
 8. An electronicdevice as claimed in claim 1 characterized in that this antiresonantcircuit is tuned to correspond to the center frequency of thetransmission band.
 9. An electronic device as claimed in claim 1,characterized in that the band separating unit is a triplexer, and inthat the device is further provided with a third band-pass filterbetween the antenna port and a third port, which third band-pass filterincludes a ladder circuit with shunt and series elements, of whichseries elements at least a first element comprises a film bulk acousticwave resonator and is provided with a parallel inductor, and wherein afurther frequency tuning element is present between the antenna port andthe third band-pass filter.
 10. An electronic device as claimed in claim1 characterized in that the parallel inductor is integrated in the bulkacoustic wave resonator.
 11. An electronic device as claimed in claim 1,characterized in that the frequency tuning element comprises aninductor; the bulk acoustic wave resonators are provided in the form ofsilicon dice, and the inductors are integrated on a substrate carrier onwhich the silicon dice of the bulk acoustic wave resonators are alsomounted.
 12. Method of isolating an Rx-band and a Tx-band in a duplexercomprising a transmit port, a receive port and an antenna port, and afirst baud-pass filter connecting the transmit port and the antennaport, wherein the first band-pass filter includes a first ladder circuitwith shunt and series elements and a series arrangement connects thereceive port and the antenna port, wherein the series arrangementcomprises a second band-pass filter including a second ladder circuitwith shunt and series elements, wherein the elements of both the firstand the second band-pass filter comprise a film bulk acoustic waveresonator, the method being characterized in that an antiresonantcircuit blocks the transmission band at the entrance of the secondband-pass filter, as its antiresonant frequency corresponds to thecenter frequency f_(Tx) of the transmission band and a resonant circuitallows the receive band to pass, as its resonant frequency correspondsto the center frequency f_(Rx) of the receive band.